Replacement of nucleotides in transfer RNA will be effected by a combination of chemical and enzymatic techniques to facilitate the study of the molecular mechanism of the partial reactions of protein biosynthesis. For example, by replacement of the nucleoside at the 3'-terminus of the tRNA with 2' - or 3'-deoxyadenosine, it should be possible to determine the positional specificity for the amino acid moiety on aminoacyl-tRNA during certain of the partial reactions. Replacement of nucleotides within the primary sequence, giving tRNA's modified internally at single sites, should help to define the roles of individual tRNA components in protein biosynthesis and in other tRNA-mediated processes. Additionally, the modified species should be of utility in studies of tRNA processing during biosynthesis, e.g., in specifying the structural parameters that facilitate removal of the intervening sequences during the processing of certain eukaryotic tRNA precursors.